1. Technical Field
The present invention relates to a semiconductor light emitting device (LED) that has high light extraction efficiency. More particularly, the present invention relates to an LED that has a two-dimensional periodic structure and has an asymmetric refractive index distribution with respect to the interposed active layer. The present invention further relates to a light emitting device (LED) that has high light extraction efficiency, has a two-dimensional periodic structure, and has an intermediate layer arranged on the light extraction side of the active layer. The intermediate layer has a refractive index that is lower than or equal to that of the active layer, but is the highest of all of the refractive indices of the semiconductor layers arranged on the light extraction side of the active layer.
2. Description of the Related Art
Semiconductor-based light emitting devices (LEDs) are expected to be used in a wide range of applications, including signs, displays and illuminations. Semiconductor materials generally have a higher refractive index than the surrounding medium, such as air and resins. Thus, the light emitted from an LED undergoes total internal reflection and is not effectively extracted outside. This leads to a problem that the efficiency of light utilization by LEDs is low. For example, semiconductor materials typically have a refractive index of 2.0 to 3.5 while the surrounding medium such as air and resins has a refractive index of 1.0 to 1.5. For this reason, the light emitted from a semiconductor material undergoes total internal reflection at the boundary of the semiconductor and the surrounding medium, allowing only a small percentage of the emitted light (i.e., a few percent) to escape from the LED.
Thus, a way is needed to effectively extract the light emitted from LEDs to the outside.
One approach is to form a periodic structure in the surface of a semiconductor (see, for example, U.S. Pat. No. 5,779,924, Japanese Patent Application Laid-Open No. Hei 10-4209, Japanese Patent Application Laid-Open No. 2004-128445, and Japanese Patent Application Laid-Open No. 2004-31221). The periodic structure in the surface of the semiconductor serves to change the wavenumber of the internal light and thus, its direction, so that the internal light can no longer undergo the total internal reflection and can thus be extracted into the surrounding medium. In this technique, the large solid angle of the internal light improves the extraction efficiency of light.
A three-dimensional light wave simulation has revealed that the extraction efficiency of self-luminous devices featuring the above-described periodic structure is limited by the diffraction efficiency of the periodic structure: A significant amount of light is left trapped within LEDs. The three-dimensional light wave simulation is a known wave optics-based simulation technique developed by the present inventors (see Japanese Patent Application Laid-Open No. 2005-69709).
One problem of the periodic structure approach is that the structure may not be made with perfect periodicity depending on the type of the process used to make it. Such defective periodic structures cannot achieve sufficiently high light extraction efficiency. Also, making the periodic structure with perfect periodicity requires an elaborate process, which can add to cost.
One approach to improve the light extraction efficiency is to integrate the two-dimensional periodic structure into the light emitting layer (active layer). Although the integration of the two-dimensional periodic structure significantly improves the light extraction efficiency, the quality of the light emitting layer can significantly be affected. Thus, this approach still remains impractical.
Semiconductor surfaces containing broken bonds also contain various surface states and defects within the band gap. Carriers present near the semiconductor surface thus recombine through these surface states and defects (surface recombination). A two-dimensional periodic structure formed on the active layer creates on the processed surface a state similar to that seen on the surface in which bonds between crystals are broken. As a result, carriers injected into the active layer recombine at the surface, producing heat rather than light. This decreases the light conversion efficiency.